AQA biology AS topic


Biological Molecules

Bonding and the formation of molecules

  • Covalent Bonding - atoms share a pair of electron in their outer shell. As a result both full outer shell, stable, molecule formed
  • Ionic Bonding - ions with opposite charge attracts one another. The electrostatic attraction is known as an ionic bond
  • Hydrogen Bonding - the electrons within a molecule are not evenly distributed but tend to spend more time at one position. This region is more negatively charged than the rest of the molecule. This molecule is polarised and is a polar molecule. The negative region and positive region attract each other. A weak electrostatic bond is formed between the two. Each individually weak collectively form important forces which alter physical properties. (true for water)

Polymerisation, condensation, hydrolysis 

  • Monomers + Monomers = Polymer 
  • Process called polymerisation
  • Condensation reaction - reaction that produces water.  
  • Hydrolysis reaction - water is used to break bonds

Metabolism and moles 

  • Metabolism - all chemical processes that take place in living organisms
  • Mole - unit for measuring the amount of a substance (mol). 
  • A mole is the molecular mass in grams 
  • Molar solution (M) - solution that contains one mole of solute in each litre of solution

Life based on carbon

  • Readily form bonds with other carbon atoms 
  • allows long sequence of carbon atoms to be made 
  • forms a backbone where other atoms can attach 
  • carbon-containing molecules = organic molecules
  • life based on small number of chemical elements

Making of large molecules 

  • Carbohydrates and proteins are polymers 
  • Most molecules  made up of 4 elements: carbon, hydrogen, oxygen and nitrogen.
  • In carbohydrates = single monomer called a monosaccharide 
  • monosaccharide + monosaccharide = disaccharide 
  • many monosaccharides form polysaccharides


  • sweet soluble substances
  • general formula (CH2O)n where n from 3-7 
  • Examples: Glucose, Galactose and Fructose. 
  • Glucose C₆H₁₂O₆, is a hexose sugar and has two isomers α glucose and β glucose

Test for reducing sugars

  • All monosaccharides and some disaccharides are reducing sugars. 
  • This means that they are able to donate electrons to another molecules.
  • Benedict’s reagent is a solution of copper(II) sulfate. When heated with a reducing sugar, the copper(II) sulfate accepts electrons from the sugar forming an insoluble red precipitate of copper(I) oxide.
  • Add 2cm3 of food sample to be tested in liquid form 
  • add equal volume of Benedict’s reagent 
  • heat mixture gently in water bath for 5 minutes 


  • monosaccharides + monosaccharides = disaccharides condensation reaction occurs 
  • glycosidic bond forms.
  • Disaccharide + water = 2 monosaccharides, hydrolysis reaction, breaks glycosidic bond 
  • Glucose + Glucose = Maltose
  • Glucose + Galactose = Lactose
  • Glucose + Fructose = Sucrose

Test for non-reducing sugars

  • Sucrose = non-reducing sugar = don't change colour of Benedict’s reagent when heated with it 
  • has to be hydrolysed into its monosaccharide 
  • add 2cm3 of food sample into test tube with Benedict’s reagent and filter 
  • place test tube in boiling water bath fro 5 minutes, no colour change = no reducing sugar present 
  • add another 2cm3 of food sample to 2cm3


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